Due to desirable advantages in higher transmission rate, resistance to multi-path interferences and so on, the OFDM (Orthogonal Frequency Division Multiplexing) technology is chosen as the modulation scheme for some digital broadcast systems, for example, DAB (Digital Audio Broadcasting) and DVB-T (Digital Terrestrial Video Broadcasting) systems. Typically, an OFDM signal comprises one or more OFDM symbols. To suppress ISI (Inter-Symbol Interference) suffering from multi-path propagation, a method based on GI (Guard Interval) may be employed, that is, to insert a GI having a certain duration before each OFDM symbol and copy the last portion of the OFDM symbol to form the content of the GI.
In a paper entitled “Low-complex Frame Synchronization In OFDM System” by J-J. van De Beek, M. Sandell, M. Isaksson, and P. O. Borjesson, Proceeding of ICUPC'95, Tokyo, 1995, pp. 982-986, J-J. van De Beek et al proposed a method of detecting symbol synchronization information for an OFDM signal by using the GI. FIG. 1 is a block diagram showing a receiver implemented in accordance with the method of detecting symbol synchronization information for an OFDM signal with GI as proposed by J-J. van De Beek et al. As shown, the receiver 100 comprises a correlator 110, a moving sum unit 120, an absolute value calculator 130 and a detector 140. The correlator 110 comprises a delayer 112, a conjugate calculator 114 and a convolver 116.
In environments where a large channel spread exists, for example, an SFN (Single Frequency Network) defined in the Nordig specification and a channel with 0-DB echo, it will be hard to detect symbol synchronization with the method proposed by J-J. van De Beek et al. In a paper entitled “Enhanced Symbol Synchronization Method for OFDM system in SFN Channels,” by Arto Palin and Jukka Rinne, GLOBECOM'98, Vol. 5, November 1998, pp. 2788˜2793, Arto Palin and Jukka Rinne presented the reasons for that. For the purpose of understanding, the reasons are simplified as follows.
FIG. 2 is a diagram schematically illustrating the impulse response for a channel with a large channel spread. In FIG. 2, a first multi-path signal δ(n) arrives at instant 0, and another multi-path signal δ(n−τ) arrives at instant τ, where τ may have a large value. For example, in an SFN channel defined by version 1.0.2 of the Nordig specification, τ has a range of [1.95 μs,0.95*DGI], where DGI is the duration of the GI. Table.1 lists some typical values for the GI defined in the Nordig specification.
TABLE 1Typical values for the GIThe protected channelSize of FFTDuration of GI (μs)spread (μs)2K FFT, GI = 1/327.01.95~6.652K FFT, GI = 1/1614.01.95~13.32K FFT, GI = ⅛28.01.95~26.62K FFT, GI = ¼56.01.95~53.28K FFT, GI = 1/3228.01.95~26.68K FFT, GI = 1/1656.01.95~53.28K FFT, GI = ⅛112.01.95~106.48K FFT, GI = ¼224.01.95~212.8
FIG. 3 shows a process for forming a correlation signal after propagation through a channel with a large channel spread as shown in FIG. 2 with the method proposed by J-J. van De Beek et al. As shown, signal r(n) is a signal received via the first multi-path, or a signal generated by convolving the transmitted signal with the first multi-path signal δ(n); while signal r(n−τ) is a signal received via another multi-path, or the received signal generated by convolving the transmitted signal with δ(n−τ). Signals r(n) and r(n−τ) are superimposed together to form the received signal received at the receiver. Illustratively, it can be seen that signal r(n) contains a GI and an OFDM symbol x(n), wherein the GI has the same length and content as the last portion of x(n). Signals r(n−N) and r(n−τ−N) may be obtained at a delayer 112, where N is generally the duration of an OFDM symbol and may be regarded as the size of IFFT (Inverse Fast Fourier Transform) performed at the corresponding OFDM transmitter. After correlating and moving summing signals r(n) and r(n−N), and calculating the absolute value for the real component of each sample, a correlation signal C0(n) may be obtained. After correlating and moving summing signals r(n−τ) and r(n−τ−N), and calculating the absolute value for the real component of each sample, a correlation signal Cτ(n) may be obtained. The correlation signals C0(n) and Cτ(n) each have a peak. For a correlation signal having only one global peak, the detector generally may obtain the starting point of symbol synchronization by detecting the peak of the correlation signal. However, signals r(n) and r(n−τ) are superimposed, which causes the addition of the correlation signals C0(n) and Cτ(n), to form a combined correlation signal C0+τ(n). For ease of understanding, it's assumed here that the channel has no interference, so the correlation signals C0(n) and Cτ(n) have an ideal triangle, which causes the combined correlation signal C0+τ(n) obtained by addition to not have the only peak any more, but have a platform having a certain length and the only value, whose starting point and ending point have the same positions as the peaks of the correlation signals C0(n) and Cτ(n), respectively. In a practical system, however, due to the existence of interferences, the correlation signal C0+τ(n) may have multiple small sharp peaks, i.e., local peaks, instead of a platform having a certain length and the only value. For this reason, it is difficult for the detector to detect the synchronization point precisely, and hence it becomes hard or even impossible to achieve symbol synchronization.
To solve this problem, A. Palin and J. Rinne proposed a method for double correlation, as shown in FIG. 4. In a receiver 400 implementing the double correlation method, a second correlator 430 and a second moving sum unit 440 are inserted after a first correlator 410 and a first moving sum unit 420 and before an absolute value calculator 450. The second correlator 430 comprises a delayer 432, a conjugate calculator 434 and a convolver 436, where the delayer 432 delays the input signal of the second correlator 430 by (N+L) samples. After two correlations and moving sums, conventional peak detection methods may be used to detect the symbol synchronization point.
The double correlation method may reduce influences from noises and interferences. However, as admitted in the paper entitled “Enhanced Symbol Synchronization Method for OFDM system in SFN Channels”, the double correlation method only applies to channels whose channel spread is less than or equal to half the length of the GI.
It is, therefore, necessary to provide a method to enable precise detection of symbol synchronization information for an OFDM signal based on GI, without being limited by the channel spread for the propagation channel.